1. Field of the Invention
This invention relates to a reflection-type liquid crystal display device and, particularly, to a reflection-type liquid crystal display device used for low-power-consumption units such as portable terminals and having a feature in the constitution for suppressing a decrease in the contrast caused by the reflection on the interface without decreasing the quality of display.
2. Description of the Related Art
Owing to their features such as small in size, light in weight and low power consumption, the liquid crystal display devices have been widely used as data equipment terminals, TVs, portable data equipment terminals, display monitors such as video cameras, etc.
The liquid crystal material does not emit the light by itself and a source of light is necessary when it is used as a display device. In the case of a reflection-type liquid crystal display device used as a low-power-consumption unit such as a portable terminal, in particular, the indoor illumination serves as a source of light or a front light unit is used as a source of light (see, for example, JP-A-2001-108986).
Here, a conventional reflection-type liquid crystal display device will be described with reference to FIG. 17.
FIG. 17 is a sectional view schematically illustrating a conventional reflection-type liquid crystal display device comprising a front light unit 90 and a liquid crystal display panel 80 constituted by a liquid crystal layer 82 held between a TFT substrate 81 and a CF (color filter) substrate 83, which are firmly held face to face by a frame 85 maintaining a small gap or an air layer 96.
The front light unit 90 includes a source 91 of light, a light guide plate 93 and a reflector 92 for reflecting and collecting light from the source 91 of light toward the light guide plate 93. A prism 94 is engraved in the surface of the light guide plate 93, i.e., on the side of the viewer, and light being guided is partly reflected toward the liquid crystal display panel 80.
Usually, further, a reflection prevention film 95 is provided on the back surface of the front light unit 90 and a polarizing plate 84 is provided on the front surface side of the liquid crystal display panel 80.
The pitch of the prism 94 is so set relative to the pixel pitch of the liquid crystal display panel 80 that moiré fringes appear little.
For example, the prism pitch is set to be equal to the pixel pitch so that the moiré pitch becomes an infinity or is so set that the moiré pitch becomes very fine.
In general, further, the liquid crystal display panel 80 is so constructed as to elevate the reflection factor in order to realize bright display. In the use of small devices such as cell phones and PDAs, in particular, the designing aims at accomplishing a very high reflection factor since the viewing angle need not be broad.
In this reflection-type liquid crystal display device, a circularly polarizing plate is usually used, the circularly polarizing plate being made up of a retardation plate and a polarizing plate. When no retardation is occurred in the liquid crystal display panel or when light is reflected in front of the liquid crystal display panel, however, the reflected light is absorbed by the circularly polarizing plate and does not go out.
This is because the circularly polarizing plate converts, first, the incident light into linearly polarized light through the polarizing plate and converts, next, the light into circularly polarized light through the retardation plate. The circularly polarized light reflected by the interface falls again on the retardation plate and is converted into linearly polarized light with its direction of polarization being turned by 90 degrees. The reflected light which is the linearly polarized light turned by 90 degrees is absorbed by the polarizing plate and does not go out.
The structure for arranging the circularly polarizing plate can be contrived in the following three types as illustrated in FIG. 18.
FIG. 18A is a view schematically illustrating the constitution of when the circularly polarizing plate is stuck to the side of the liquid crystal display panel, in which the circularly polarizing plate 100 comprising a polarizing plate 101 and a retardation plate 102 is stuck onto the display surface of the liquid crystal display panel 80 by using a sticking agent 103, and an air layer 96 exists relative to the light guide plate 93.
FIG. 18B is a view schematically illustrating the constitution of when the circularly polarizing plate is stuck to the side of the light guide plate, in which the circularly polarizing plate 100 comprising the polarizing plate 101 and the retardation plate 102 is stuck onto the back surface of the light guide plate 93 by using the sticking agent 104, and the air layer 96 exists relative to the liquid crystal display panel 80.
FIG. 18C is a view schematically illustrating the constitution of when both surfaces of the circularly polarizing plate are stuck, in which the circularly polarizing plate 100 comprising the polarizing plate 101 and the retardation plate 102 is stuck onto the back surface of the light guide plate 93 and to the liquid crystal display panel 80 by using the sticking agents 103 and 104. In this case, there exists no air layer.
FIG. 19 is a diagram illustrating the reflected light components on the interfaces to the air layer.
The air layer 96 in the above constitution has a refractive index n2=1, and constituent members such as the liquid crystal display panel 80, circular polarizing plate 100, light guide plate 93 and sticking agents 103, 104 have refractive indexes n1 and n3 of about 1.4 to about 1.6. Therefore, the greatest difference in the refractive index exists on the interfaces among the constituent members and the air layer 96, and the incident light 105 is greatly refracted by the interfaces.
The reflection increases on the interface as the refraction increases. As the refraction exceeds a critical angle, further, a total reflection takes place and the contrast decreases.
In the constitution illustrated in, for example, FIG. 18A, the contrast is only about 5 to about 10. The low contrast narrows the range of reproducing colors, and the quality of display is very poor.
In principle, therefore, the constitution of FIG. 18C is desired. However, when the light guide plate 93 and the liquid crystal display panel 80 having different coefficients of thermal expansion are stuck together, there occurs peeling due to thermal shock. Or, when the rigid bodies are stuck together, air bubbles tend to infiltrate. Therefore, the use of this constitution involves difficulty except the small devices.
It has therefore been proposed to use the constitution of FIG. 18B in which the circularly polarizing plate 100 is stuck to the light guide plate 93, enabling the light 106 reflected by the interface to the air layer 96 to be absorbed by the polarizing plate 101 that constitutes the circularly polarizing plate 100 to thereby enhance the contrast (see, for example, JP-A-11-259007).
However, the above constitution of FIG. 18B is accompanied by a problem of conspicuous interference rainbow due to moiré fringes between the prism 94 of the light guide plate 93 and the pixels of the liquid crystal display panel 80, and due to interference between the reflection structure of the liquid crystal display panel 80 and the pixels.
The cause is attributed to a decrease in the diffusing light component stemming from a decrease in the angle of light guided through the light guide plate 93 as a result of sticking the circular polarizing plate 100 onto the light guide plate 93.
When the diffusing light component is strong, on the other hand, the moiré fringes and interference fringes are averaged and are weakened.
There still remains a problem of a decrease in the contrast since light 107 refracted in excess of a critical angle by the interface to the air layer 96 is totally reflected.
There further exists a problem in that the circularly polarizing plate 100 and the liquid crystal display panel 80 are abraded by the external pressure of input using a pen, and the circularly polarizing plate 100 is scarred.